1. Field of the Invention
The present invention relates to a method and apparatus for driving an inkjet head. The present invention particularly relates to a method and apparatus for driving an inkjet head, whereby the pressure in a pressure chamber is increased or reduced and ink is ejected or not ejected from a nozzle communicated with the pressure chamber by applying drive signals having predetermined drive waveforms to an actuator provided with the pressure chamber.
2. Description of the Related Art
An inkjet recording apparatus ejects ink droplets selectively from a plurality of nozzles formed on an inkjet head, to record an image on a sheet of paper. Therefore, some of the nozzles may not eject the ink for a long time. When the ink is not ejected for a long time, the ink forming meniscus in the nozzles dries out, increasing the viscosity of the ink. Such increase in the ink viscosity causes an ink ejection failure and consequently generates an abnormal image.
Japanese Patent Application Publication No. 2004-209843 discloses the technology for preventing such local increase of the ink viscosity in a meniscus part, wherein a minute drive pulse is applied at specific timing to an actuator that ejects the ink, to slightly oscillate the meniscus. More concretely, Japanese Patent Application Publication No. 2004-209843 proposes applying a minute drive pulse after an ejection pulse for ejecting the ink, so as to slightly oscillate the meniscus.
The inkjet recording apparatus often records the tones obtained by a plurality of types of dots in different sizes, in order to improve the quality of an image. In addition, as a method for changing the size of each dot, there is a known method for changing the size of each dot in accordance with the number of ejection pulses applied to the actuator. This method, for example, configures a series of drive signals by including three ejection pulses within one recording cycle, the ejection pulses being generated at regular intervals, and, when large dots are formed, all of the three ejection pulses are applied within the recording cycle. When medium-sized dots are formed, the method applies two of the three ejection pulses within the recording cycle to the actuator. When small dots are formed, the method applies one of the three ejection pulses within the recording cycle to the actuator. However, selectively applying the ejection pulses in this manner causes variations in the application intervals when recording the same dots continuously, and causes the ink droplets to deviate the courses thereof or causes fluctuations in the amount of the ink droplets, destabilizing the ejection of the ink.
Japanese Patent Application Publication No. 2002-103619 proposes the technology for configuring a drive signal applied to an actuator, from a basic ejection pulse generated every t cycle and an auxiliary ejection pulse generated after a lapse of ½ of the cycle t since the generation of the basic ejection pulse, when changing the sizes of the dots in accordance with the number of ejection pulses. In this technology, the waveforms of the basic ejection pulse and the auxiliary ejection pulse are formed into the same waveform, and at least three of the basic ejection pulses and at least one auxiliary ejection pulse are included in a recording cycle. By applying the appropriately selected basic ejection pulses and the auxiliary ejection pulses to the actuator, the application intervals for applying the ejection pulses can be made constant, across the recording cycle, in order to record the same size of dots consecutively.
The method described in Japanese Patent Application Publication No. 2004-209843 can prevent the ink from drying out but cannot prevent the ink droplets from deviating the courses thereof or stably eject the ink droplets.
The method described in Japanese Patent Application Publication No. 2002-103619, on the other hand, can prevent the ink from deviating its course but cannot prevent the ink from drying out.
Although it is possible to make up these disbenefits by combining these methods, in such a case, the waveforms of the pulses become long and the ink droplets cannot be ejected at a high frequency.